A liquid crystal display (LCD) is a thin, flat display device made up of any number of monochrome pixels arrayed in front of a light source or reflector. Each pixel of an LCD consists of a layer of perpendicular molecules aligned between two transparent electrodes, and two polarizing filters, the axes of polarity of which are perpendicular to each other. Typically, the surfaces of the electrodes in contact with the liquid crystal material are treated to align the liquid crystal molecules in a particular direction.
Before applying an electric field, the orientation of the liquid crystal molecules is determined by the alignment at the surfaces. In a twisted nematic device (the most common liquid crystal device), the surface alignment directions at the two electrodes are perpendicular. The molecules arrange themselves in a helical structure or twist. Because the liquid crystal material is birefringent, light passing through one polarizing filter is rotated by the liquid crystal helix as it passes through the liquid crystal layer, allowing it to pass through the second polarized filter. Half of the light is absorbed by the first polarizing filter, but otherwise the entire assembly is transparent.
When a voltage is applied across the electrodes, a torque acts to align the liquid crystal molecules parallel to the electric field, distorting the helical structure reducing the rotation of the polarization of the incident light, and the device appears gray. If the applied voltage is large enough, the liquid crystal molecules are completely untwisted and the polarization of the incident light is not rotated at all as it passes through the liquid crystal layer. This light will then be polarized perpendicular to the second filter, and thus be completely blocked and the pixel will appear black. By controlling the voltage applied across the liquid crystal layer in each pixel, light can be allowed to pass through in varying amounts, correspondingly illuminating the pixel.
LCDs like those found in calculators have direct driven image elements where a voltage can be applied across one segment without interfering with other segments of the display. These LCDs are relatively cheap to manufacture and therefore are used in low cost items such as digital clocks, watches, calculators, and toys. While cheap these LCDs are considered by some to be less than aesthetically pleasing; whereas, color LCDs are cost prohibitive for use in such devices.